The present invention relates to the field of fluorinated hydrocarbons and, more particularly, has as its subject the purification of 1,1,1,2-tetrafluoroethane.
This compound, known in the profession under the name F134a, is especially intended to replace dichlorodifluoromethane (F12) currently used as a refrigerating fluid but suspected of contributing to the depletion of the stratospheric ozone layer. In order to do this, F134a must satisfy quality standards with respect to the presence of a priori toxic impurities, such as chlorofluorinated olefins. These include in particular 1-chloro-2,2-difluoroethylene (F1122) which, given its boiling point (xe2x88x9217.7xc2x0 C.), proves to be very difficult to completely remove from F134a (B.p.=xe2x88x9226.5xc2x0 C.) by simple distillation, especially under pressure.
Now, one of the industrial syntheses of F134a consists of a gas phase catalytic fluorination of 1-chloro-2,2,2-trifluoroethane (F133a) which always produces variable quantities of F1122 as a by-product by a dehydrofluorination side reaction.
In order to solve this problem, various techniques have already been proposed. Thus, the U.S. Pat. No. 4,158,675 describes a process for in-line treatment consisting in reacting the gases resulting from the main reaction:
F133a+HF⇄F134a+HCl
in a second reactor maintained at a lower temperature than that of the main reaction. From a gas mixture whose F1122 content, relative to the organic compounds, is 5300 vpm (volume per million), the in-line treatment at 160xc2x0 C. leads to a F1122 value of 7 vpm. The major disadvantage of this process lies in the necessity of treating a significant gas flow rate and thus having a high reaction volume, which leads to a prohibitive investment and a prohibitive maintenance cost. Moreover, the lifetime of the catalyst (bulk chromium oxide) is not mentioned and, to compensate for the loss in catalytic activity, it may prove to be necessary to progressively increase the temperature, which may have inter alia the immediate consequence of a partial retrogradation of F134a to F133a by reaction with HCl. Moreover, the presence of HCl in the gases also risks causing a corrosion problem.
The patent application EP 0,467,531 describes a process for distilling azeotropic HF/F134a/F1122 mixtures. The complexity of this process and the high energy consumption which it requires do not make it particularly attractive.
Another technique, described in the patent application EP 0,446,869, consists in carrying out the synthesis of F134a from trichloroethylene and HF in the gas phase in two reactors arranged in series. In the first, F133a is converted at high temperature to F134a accompanied by F1122; the flow leaving this first reactor, to which trichloroethylene is added, passes through a second catalytic reactor at a lower temperature in order to convert the trichloroethylene and F1122 to F133a. The major disadvantage of such a process lies in the low productivity of the second reactor, limited by the removal of the heat produced by the reaction, on the one hand, and by the obligation to work with a high contact time in order to achieve very low F1122 values, on the other hand. Another disadvantage of this technique lies in the risk of retrogradation of F134a to F133a in the presence of HCl as soon as the temperature is raised too high. This obligation to perfectly control the temperature then leads to a risk of the appearance of unconverted F1122 since the equilibrated reaction:
F133a⇄HF+F1122
depends not only on the HF/F1122 and F133a/F1122 molar ratios but also on the temperature and the pressure. In this respect, there may be mentioned the patent EP 36123 relating to the catalytic addition of HF to (chloro)fluorinated olefins, especially pure F1122 (Example 4), whose fluorination at a controlled temperature (120 to 131xc2x0 C.) allows unconverted F1122 to remain at the reactor outlet.
As other means of removing olefins, there may be mentioned:
Catalytic hydrogenation (patent application WO 90/08750): this method requires expensive catalysts (precious metals) and leads to hydrogenated saturated compounds other than F133a which have to be separated subsequently; moreover, the hydrogen always entrains a small amount of F134a by vapor pressure.
Physical adsorption on a mixture of oxides of manganese and of copper (hopcalite): the major disadvantage of this process, described in the patent application EP 370,688, lies in the requirement to regularly regenerate this solid adsorbent after use; this leads to losses of F134a by more or less selective adsorption and a significant concentration of olefin during the regeneration cycle.
Extractive distillation (patent application EP 472,391) of F134a/F1122 mixtures using suitable solvents (trichloroethylene, perchloroethylene, and the like): the main disadvantage of this technique lies in the complexity of the equipment (additional distillation columns to recycle the purified solvent), the high effect of the energy cost (successive evaporations) and the more or less efficient yield of the processing.
To avoid the disadvantages of the abovementioned techniques, the present invention provides a particularly effective and economic means for purifying crude F134a containing unsaturated impurities.
The process according to the invention consists in treating a gaseous mixture of crude F134a and HF in the gas phase, in the absence of hydrochloric acid, at a temperature of between 200 and 380xc2x0 C. and under a pressure ranging from atmospheric pressure to 2.5 MPa, in the presence of a fluorination catalyst, the HF/F134a molar ratio being between 0.05 and 0.5.